Phenalenyls, Cyclopentadienyls, and Other Carbon-Centered Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 109

(a) (b) (c)

Figure3.31 Spindensitydistributionsoftheradicaldianionsof(a) 3OPO ,(b) 4OPO ,and(c) 6OPO systems.

Ar Ar

2

O O O

O OH O O

1

3

ArCH 2 COOH oxidation

s -dimer

R R

R R R R

R=H, tert -Bu

Ar = Ph, p -MeOC 6 H 4 ,
quinolyl, thienyl,
5'- tert -Bu-thienyl

Scheme 3.9 Synthetic method for 3-hydoxyphenalenone derivatives and their σ -dimerization reactions

upon oxidation

of 3OPO (Figure 3.29). These chemical reactivities were extensively investigated with various substituents
of the 3OPO system, indicating a general trend in the formation of the closed shell σ dimers at the 2-

positions. Pyrolysis of the dimer gives the corresponding neutral radical of 3OPO in solution, although

this radical could not be isolated. 40
For the stabilization of 3OPO , suppression of reactivity at the 2-position is the most effective approach.
Thus, chemical modification for the decrease in spin density at the 2-position by spin delocalization into the
π

σ

system substituted at 2-positions was investigated. Phenyl, p -methoxyphenyl, and quinolyl derivatives
result in -dimeric structures between the carbon atoms at the 2-positions (Figure 3.32a,b). 39a σ In sharp

contrast, in the σ dimer of the thienyl derivatives, the carbon atoms on the thiophene rings take part in bond

(a) (b) (c) (d)

Figure 3.32 -Dimeric structures of (a) phenyl, p-methoxyphenyl, and quinolyl derivatives, (b) X-ray crystal

structure of p-methoxyphenyl derivative. (c) and (d)

σ

dimeric structures of thienyl derivatives. (b Reprinted
from[39a],Copyright2005,withpermissionfromElsevier.)

σ